Randomized phase and amplitude radar codes for space object tracking

ABSTRACT

A method of tracking objects using a radar, includes sending a beamcode to at least one radar antenna to set a predetermined direction, using samples from a random distribution of at least one of a phase or an amplitude to generate a tracking signal pulse train, transmitting the pulse train from the at least one antenna within a pulse time window, receiving return signals from objects at the at least one antenna, and using the return signals to gather data to track the objects. A radar system has at least one radar antenna to transmit a tracking signal, a memory to store a set of random distributions, a controller connected to at least one radar antenna and the memory, the controller to execute instructions to determine which random distribution to use, generate a pulse train using the random distribution, transmit the pulse train to the at least one radar antenna as the tracking signal, and gather measurement data about objects returning signals from the tracking signal.

BACKGROUND

Some radars detect and track space objects, such as space debris andsatellites, primarily in Low Earth Orbit (LEO). Low Earth Orbittypically refers to distances from the Earth's surface to 2,000kilometers away, but that is just an example with no limitation to thatdefinition intended. Most satellites and the International Space Stationoperate in LEO, which makes tracking of debris and other objects evenmore important.

To improve detectability of the objects and measure them with high rangeand Doppler resolution, the desired operation would be integratedmeasurements of the space object over long periods of time. High Dopplerresolution means that the radar can detect objects that travelrelatively close together. In this context, long periods of time may runseveral hundreds of milliseconds or longer. These long periods of timeconflict with the transmission limitations required to resolve thedistance to the target, which may range from a few hundred to a fewthousand kilometers.

Many radars have functional constraints, including pulse duration, dutycycle limitations, minimum transmission and/or reception periods, etc.Achieving the long periods of time for measurement integration foraccurate tracking becomes difficult, if not impossible for some radars.

SUMMARY

One embodiment is a method of tracking objects using a radar, thatincludes sending a beamcode to at least one radar antenna to set apredetermined direction, using samples from a random distribution of atleast one of a phase or an amplitude to generate a tracking signal pulsetrain, transmitting the pulse train from the at least one antenna withina pulse time window, receiving return signals from objects at the atleast one antenna, and using the return signals to gather data to trackthe objects.

Another embodiment is a radar system having at least one radar antennato transmit a tracking signal, a memory to store a set of randomdistributions, a controller connected to the at least one radar antennaand the memory, the controller to execute instructions to determinewhich random distribution to use, generate a pulse train using therandom distribution, transmit the pulse train to the at least one radarantenna as the tracking signal, and gather measurement data aboutobjects returning signals from the tracking signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a radar system.

FIG. 2 shows a timing diagram for a pulse train transmission fortracking.

FIG. 3 shows a diagram of an embodiment of a waveform compared to acurrent waveform.

FIG. 4 shows a flowchart of a method of transmitting trackingtransmissions as pulse trains.

FIG. 5 shows a graph of a number of pulses integrated over time againsta graph of efficiency for a pulse train.

FIG. 6 shows a graph of a power spectral density of a tracking pulsetrain against a graph of an expanded view or a portion of the powerspectral density.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As used here, the term “radar” means any system used to perform radiodetection and ranging. It may take the form of a parabolic reflector,planar array, phased antenna arrays, and other examples, having atransmitter and a receiver. An “antenna” consists of the radiatingelement or elements that radiate radio signals generated by thetransmitter and receives radio signals and pass them to the receiver.

A “beamcode” is a signal sent to at least one antenna to provide apredetermined direction in which a transmission is to be sent. A“tracking signal” or “tracking transmission” is a signal sent by theantenna into the predetermined direction with the intent of receivingreturn signals from objects in free space into which the trackingtransmission was sent. A “return signal” is a signal caused by thetracking signal reflecting off an object in free space and returned tothe antenna.

The embodiments in here employ pulses sampled from uniform, random“distributions” as the tracking or transmitted signal. The randomsamples may also be referred to as a “code,” or a “waveform.” Someembodiments may use non-uniform, random distributions.

FIG. 1 shows an embodiment of a radar system 10 usable with the methodsand techniques discussed here. One should note that the elements heremay have several different configurations but are shown in thisarrangement for ease of discussion. The radar system has a controller12, which may reside inside the radar transmitter, receiver, or acombined transmitter/receiver. The radar may consist of a phased arrayof antenna elements, where each element has its own controller, or theelements are divided into sub-arrays, each sub-array having acontroller.

The controller may include dedicated control hardware or components 16for controlling the antenna array and transmitter/receiver, and aseparate computer 14 for selection of random distributions used by theembodiments here. The radar control and the computer may reside in thesame device, or separate devices, within the system. Similarly, theembodiments here use random distributions that may be stored in a memorysuch as 20. These may be stored in a separate memory 22 from the memoryused for transmission and reception 24, or they may share a commonmemory.

Typically, radars used for tracking low earth objects generate a seriesof periodic pulses sent into space. As will be discussed below,different radars have different performance parameters. Examples ofradar systems with the specific performance parameters will be givenbelow, but performance parameters may include a maximum pulse width, aminimum pulse width, duty cycle, beamcode times, beamcode wait times,transmit/receiving padding, individual pulse length, interpulse period(IPP), among others.

In one example, a typical pulse may have a duration between 10microseconds and 2 milliseconds. In current radars, one pulse is sentduring that time, and several pulses are integrated over several pulsewidths to allow reception of a number of return signals to provideaccurate tracking. In contrast, the embodiments here use a pulse traintransmitted according to a uniform, random distribution. This allowsmultiple pulses of received returns to be integrated.

FIG. 2 shows an example of such a pulse train. The top line shows thebeamcode transmission. Each beamcode corresponds to a single lookdirection. In FIG. 2, the beamcode BM is transmitted at the beginning ofthe pulse interval, t₀ until the end of the transmission time,t_(beamcode). There is then a wait time between t_(beamcode) andt_(wait) while the beamcode is processed. A pulse train is then sentbetween t_(wait) and the end of the pulse train interval t_(pt). Thepulse train interval may also be referred to as the pulse train window.

The pulse train results from a uniform, random distribution, either inpulse length or amplitude or both. In the example of FIG. 2, each pulsehas constant amplitude but is coded with a phase drawn from a uniformrandom distribution (not shown), typically polyphase. Individual pulselength and IPPs are also drawn from the random distribution.

FIG. 3 shows a comparison between a traditional waveform shown by pulses24 and the pulse train, shown by the varied pulses such as 26. Thetraditional waveform used as an example here has a 2 millisecond pulsewith a 20 millisecond IPP. The waveform shown by the randomized pulsessuch as 26 is the RF waveform. Within the transmission envelope, it iscoded with a random polyphase code (not shown).

FIG. 4 shows an embodiment of a process of generating the pulse trainand the selection of random samples. The main flow of the processrelates to the generation of the pulse train. At 30, the beamcode for apredetermined single look direction is transmitted to the antenna tocause the antenna to orient itself into the proper direction, which canbe done with either electronic, as in a phased array, or mechanicalsteering, as in a moveable antenna. At 32, a uniform, randomdistribution is used to generate a tracking signal pulse train such asthe one shown in FIGS. 2 and 3. The radar transmits the pulse train at34 and receives the return signals at 36. The reception occurscontinuously throughout the process. By using the random code to varythe phase of the transmission signal, the received signal from eachpulse can be distinguished from each other. In current systems, anambiguity exists between the signal from one transmit pulse at one rangeand the signal from another transmit pulse at another range. This isreferred to as range-aliasing. By randomly phasing each transmit pulse,the system can deduce that a returned signal with that same pattern camefrom that transmission, removing all range-ambiguity.

The random samples are selected based on examination of the waveformsidelobes. At 40, the process generates a pulse train. For example, theprocess may generate a set of random pulses within the minimum andmaximum pulse lengths, a set of random duty cycles within a minimum andmaximum duty cycles, and use these to form a pulse train.

At 42, a number of samples in the receiving time, or window, that areunusable due to transmitting is determined. A transmitting signal willsaturate any received signal that falls within the transmit window ofthe transmitting signal. This results in an efficiency measure thatequals the number of usable samples divided by the total number ofsamples. This is then used to determine the efficiency of that pulsetrain previously generated. FIG. 5 shows an example of the efficiency ofa waveform. The top graph shows the number of pulses integrated as afunction of the relative sample time, which corresponds to thetime-of-flight to a target. The solid line 49 shows the ideal case, ifall pulses were integrated. As shown in the bottom graph, the ratio ofthe number of pulses integrated to the number of pulses transmittedcorresponds to the efficiency percentage in the lower graph. Theembodiments here result in an efficiency of 80-90%, meaning that at agiven time-of-flight (range) 80-90% of the samples can be integrated formeasurement of the target.

Returning to FIG. 4, the process then determines the peak sidelobe levelat 46. Generally, this determination occurs based upon the largestsidelobe in frequency space.

The efficiency measure and the peak sidelobe level will define a metricat 48. The metric attempts to maximize the efficiency while minimizingthe peak sidelobe level. Using this metric, the process chooses the codewith the best metric in one embodiment. Another embodiment may define adesired efficiency and find the distribution that matches thatefficiency. Multiple iterations may occur until the desired efficiencyis achieved. The resulting code, or distribution, that meets thecriteria established as the desired characteristic, will then be used togenerate the transmission pulse train. A variety of search methods maybe used to find the optimal transmission sequence for a givenapplication.

In another embodiment, other parameters of the pulse train may be usedto define a metric. These parameters may include, but are not limitedto, the integrated sidelobe level, the main lobe width, and the totalpower in the main lobe

Using the embodiments here, the Doppler ambiguity caused by transmittinga regular sequence of pulses is reduced. FIG. 6 shows a comparison ofthe power spectral density of a currently-used waveform, and a waveformusing the embodiments discussed here. The top graph has a scale that is100 times the scale of the bottom graph, and provides a wider view ofthe comparison. In the top graph, the traces 50 result from theembodiments discussed here. The traces 52 result from the traditionalconstant pulse length that has a pulse length time, and IPP.

As shown in the lower graph, with a lower scale one can see that thesignal 52 has predictable Doppler sidelobes occurring at every 50 Hz(1/IPP) or every 17 microseconds. This causes severe ambiguity, meaningthat it is difficult to resolve which velocity peak the target is movingat. In the signal traces 50, the signal randomizes the sidelobes. Thisallows for approximately 13 dB of Doppler discrimination. The Dopplerresolution, or width of the mainlobe, is barely affected. This isdetermined by the total integration time ( 1/100 millisecond=3.35microseconds).

As mentioned above, the embodiments here may be applied to any radararchitecture and any set of system parameters. However, for ease ofdiscussion and understanding, two different examples are provided here.A first example has the following operating parameters. The centerfrequency resides at 440 MHz, but is adjustable as needed. The beamcodemessage is 16-bits with a first bit that is always high, and each bit is2 microseconds for a total duration of 32 microseconds. The beamcodewait time is 400 microseconds, the system waits 400 microseconds afterbeamcode transmission. The transmission from the power waveform to theradio frequency (RF) involves delays, so the system requires transmitand receive padding. In this embodiment, a lead pad consists of a 2microsecond at the beginning of each transmit and receive interval. Thisensures that the transmit/receive arrives before the RF. The trailingpad is 10 microseconds at the end of each interval. This ensures thatthe RF signal before gating turns off.

The individual pulse length is drawn from the random distributionsdiscussed above, in this example the range is from 100 microseconds to500 microseconds, but can be adjusted anywhere within the transmitlimits. The duty cycle for each pulse is drawn from a distribution, andmay range from 5% and 20% while trending towards an average duty cycleof 10%. These values could be adjusted within the range of thesolid-state power amplifier (SSPA) subject to performance limits. Thebaud length, the baud duration within a pulse, is nominally set at 0.5microseconds, 2 MHz, but could be increased. The phase for each baud isdrawn from the random distribution of the entire unit circle.Alternatively, a binary phase could also be used.

The pulse train duration is set by the maximum integration time desiredfor a single look direction. Nominally, this is set at 100 millisecondsfor this particular embodiment, but can be adjusted. This time alsodetermines the nominal Doppler resolution, a 100-microsecond integrationtime gives a 10 Hz (3.3 m/s) Doppler resolution. A non-transmit receivewindow can be left at the end of the pulse train, which shouldcorrespond to the maximum range desired. In one embodiment, this is setat 20 milliseconds. The average RF duty cycle over an entire pulse trainshould not exceed the SSPA limits, around 10% dependent upon current andtemperature.

In another embodiment, the center frequency is approximately 2950 MHz.The beamcode message is 16 bits, at 1 microsecond, for a total durationof 16 microseconds. The beamcode waiting time takes 20 microseconds. Thelead transmit/receive padding is 2 microseconds, and the trail pad is 2microseconds. The individual pulse length is a random distributionbetween 100 to 500 microseconds, adjustable as needed. Generally, longerpulses are better for unambiguous range measurements and reducedshort-range clutter but shorter pulses are better for Dopplermeasurements and decoding.

The duty cycle random distribution ranges from 10% to 30% but trendstoward an average duty cycle of 10%. The baud duration within a pulsewill be set at 0.25 microseconds (4 MHz), but can be increased. Asbefore, the pulse train duration is set by the maximum integration timedesired for a single look direction. This is initially set at 100milliseconds, but is adjustable. A 100-millisecond integration timegives a 10 Hz (0.5 m/s) Doppler resolution. Similar to the embodimentabove, the limits of SSPA, determined by current and temperature.

In this manner, a radar can transmit pulse trains in the place of thesingle pulses currently being used. This increases the efficiency of theradar and provides better resolution than current radars by reducing theDoppler ambiguity.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.

What is claimed is:
 1. A method of tracking objects using a radar, themethod comprising: sending a beamcode to a radar antenna to set apredetermined direction; using samples from a random distribution of atleast one of a phase or an amplitude to generate a tracking signal pulsetrain; transmitting the pulse train from the radar antenna within apulse time window; receiving return signals from objects at the radarantenna; and using the return signals to gather data to track theobjects, wherein at least one of: (a) wherein using a randomdistribution comprises: generating a first pulse train; counting anumber of samples in a receive time window for the radar that areunusable due to transmitting to produce an efficiency of the first pulsetrain; determining a largest sidelobe in frequency space for thereceived signals to identify a peak sidelobe; combining the peaksidelobe and the efficiency to produce a metric; and using the metric toselect a distribution to be used, or (b) wherein using a randomdistribution comprises: generating a first pulse train; determining aparameter of the pulse train using the parameter to produce a metric;and using the metric to select a distribution to be used.
 2. The methodof tracking objects of claim 1, further comprising having a wait timeafter sending the beamcode to allow for beamcode processing at theradar.
 3. The method of tracking objects of claim 1, wherein using arandom distribution comprises: generating a first pulse train; countinga number of samples in a receive time window for the radar that areunusable due to transmitting to produce an efficiency of the first pulsetrain; determining a largest sidelobe in frequency space for thereceived signals to identify a peak sidelobe; combining the peaksidelobe and the efficiency to produce a metric; and using the metric toselect a distribution to be used.
 4. The method of claim 1, wherein therandom distribution is applied to a phase of the tracking pulse train.5. The method of claim 1, wherein the random distribution is applied toan amplitude of the tracking signal pulse train.
 6. The method of claim1, wherein the random distribution is applied to a phase and anamplitude of the tracking pulse train.
 7. The method of claim 1, whereinthe pulse train is adapted to occur within a duty cycle constraint. 8.The method of claim 1, wherein the pulse train is adapted to occurwithin a pulse length time.
 9. The method of claim 1, wherein the randomdistribution is one of uniform or non-uniform.
 10. The method of claim1, wherein using a random distribution comprises: generating a firstpulse train; determining at least one parameter of the pulse train usingthe at least one parameter to produce a metric; and using the metric toselect a distribution to be used.
 11. The method of claim 10, whereinthe at least one parameter includes one of an integrated sidelobe level,a main lobe width, and a total power in the main lobe.
 12. A radarsystem, comprising: a radar antenna to transmit a tracking signal; amemory to store a set of uniform, random distributions; a controllerconnected to the radar antenna and the memory, the controller programmedto execute instructions to: determine which random distribution to use;generate a pulse train using the random distribution; transmit the pulsetrain to the radar antenna as the tracking signal; and gathermeasurement data about objects returning signals from the trackingsignal.
 13. The radar system of claim 12, wherein the controllercomprises a computing device connected to a radar controller.
 14. Theradar system of claim 12, wherein the radar antenna is one element of aphased array of elements.
 15. The radar system of claim 14, wherein eachelement of the phased array has a controller.
 16. The radar system ofclaim 14, wherein the elements of the phased array of elements aredivided into sub-arrays, each sub-array having a controller.